With recent advances in group III-based ultraviolet (UV) light emitting diode (LED) technology, interest in using UV LEDs for various applications, such as disinfection of medical tools, water purification, fluorescence spectroscopy, medical therapy, and the like, is increasing. Despite tremendous efforts, UV LEDs continue to suffer from relatively low external quantum efficiencies. Improvement in light extraction from the UV LED structure can increase the overall efficiency of a device. One approach for improving light extraction uses an index matching encapsulant (e.g., similar to the approach used for visible LEDs) in order to decrease the total internal reflection (TIR) from the device surfaces and, as a result, extract more light from the UV LED.
Typical epoxy resin materials used for visible LED encapsulation are not adequate for UV LEDs as the resins are not sufficiently transparent to UV radiation and quickly deteriorate under the UV radiation. An ideal encapsulant should be “stable.” In particular, the optical and physical properties of the encapsulant should not change during packaging, LED assembly, and during the operating lifetime of the LED. For example, an encapsulant should be resistant to heating during the LED assembly, such as during soldering a chip onto a printed circuit board or during a curing process. During the curing process, drying of the encapsulant can further induce stresses in the material. As a result, an encapsulant that is not prone to crack during the curing procedure can be selected.
A thermal coefficient of expansion (TCE) of the encapsulant can be chosen to match the TCE of an LED package in order to reduce stresses during temperature cycling, which can occur during the manufacture and operation of the LED. One approach to control TCE is by designing a composite material having the desired thermal characteristics. For example, epoxy-based optically transparent nano-composites have been studied for photonic packaging, and the effect the particle fraction of a silica filler has on the thermal coefficient of expansion of the composite material has been analyzed. TCE also has been investigated with respect to the effect of percolation in a silicon carbide (SiC) whisker reinforced ceramic composite. The effects of silica filler on the mechanical properties of a composite encapsulant have also been investigated.